The present invention relates to a current conversion device for a single- or multi-axis arrangement having a voltage-source DC link, a plurality of inverters which are each connected to the voltage-source DC link on one side and can be connected to a motor in each case on the other side, and a control device with which each of the inverters can be short-circuited. Furthermore, the present invention relates to a method for operating such a current conversion device.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Permanently excited synchronous motors (the present application is not restricted to such synchronous motors) are in most cases operated using frequency converters or inverters. Here many motors are designed such that they work in the field weakening range at high speeds. In this case the speed-dependent terminal reverse voltage of the motor is reduced by a suitable target current presetting, to the extent that the actual, uncompensated reverse voltage would lie above the maximum permissible DC link voltage of the connected DC link converter. If the frequency converter is powered down at an operating point such as this, the motor feeds power back into the DC link via the inverter diodes because of the high reverse voltage. If this generating power cannot be discharged from the DC link (e.g. active power infeed or brake chopper), the voltage of the DC link rises continuously. This can even result in destruction of the converter.
Until now there have been two strategies for limiting the DC link voltage in synchronous motors in the field weakening range.
“Voltage Protection” Module (VPM):
An additional module, called VPM, is connected to the three motor leads of the synchronous machine. Inside the module the three phases are rectified via a diode bridge and, if need be, are short-circuited via a thyristor. The thyristor is switched if the rectified voltage has reached a limiting value which cannot result in damage to the DC link. The VPM module must be able to safely manage the short-circuit current supplied by the synchronous machine. In a multiple drive group, in other words a group of converter modules, each with its own motor, each converter having a synchronous machine in the field weakening range must be assigned a VPM module. The disadvantage of the VPM is that an additional module must be employed.
Internal “Voltage Protection” (iVP):
With this method, the converter itself takes over the function of the VPM. A short-circuit of all three motor phases can likewise be achieved by selectively switching of all upper or lower power semiconductors. For the trigger mechanism it is necessary on the one hand to detect whether the parameters for an armature short-circuit are in place, i.e. the synchronous machine is located in the field weakening range and the recuperated energy cannot be taken up by the DC link. The armature short-circuit is, as with the VPM module, not triggered until a certain limiting value of the DC link voltage is reached.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved current conversion device for a single- or multi-axis arrangement, in which the inverter can be safely powered down.